ECE 477 Digital Systems Senior Design Project Rev 8/09

Homework 6: Printed Circuit Board Layout Design Narrative

Team Code Name: _____POV Machine______Group No. ___7__

Team Member Completing This Homework: ______Eric Glover______

E-mail Address of Team Member: __esglover @ purdue.edu

Evaluation:

SCORE

/

DESCRIPTION

10 /

Excellent – among the best papers submitted for this assignment. Very few corrections needed for version submitted in Final Report.

9 /

Very good – all requirements aptly met. Minor additions/corrections needed for version submitted in Final Report.

8 /

Good – all requirements considered and addressed. Several noteworthy additions/corrections needed for version submitted in Final Report.

7 /

Average – all requirements basically met, but some revisions in content should be made for the version submitted in the Final Report.

6 /

Marginal – all requirements met at a nominal level. Significant revisions in content should be made for the version submitted in the Final Report.

* /

Below the passing threshold – major revisions required to meet report requirements at a nominal level. Revise and resubmit.

* Resubmissions are due within one week of the date of return, and will be awarded a score of “6” provided all report requirements have been met at a nominal level.

Comments:

Comments from the grader will be inserted here.

1.0  Introduction

The project is an electronic display device that uses a single column of rotating LEDs turned on and off at specific time intervals utilizing the phenomenon of persistence of vision to create the illusion of a stationary image floating in the air. The POV Machine contains four separate PCBs each serving a unique function within the overall project. The four PCBs are: the spinning upper-board PCB, the LED post PCB, the stationary lower-board PCB, and the RF Transmitter PCB. The PCB layout of the RF Transmitter and the Receiver containing the phase detection hardware are the most critical components of the PCB design. Other PCB layout considerations include EMI/RFI from the microcontrollers and switching power supplies as well as mounting and packaging techniques that will require the least amount of wiring in the final package.

2.0  PCB Layout Design Considerations - Overall

The Persistence of Vision machine incorporates several different components that require consideration when faced with the task of laying the hardware out on a PCB. The first concern is the placement of each of the different circuit sections. On the lower stationary PCB, there are three main sections. These are the motor power circuitry, the 1.2GHz RF circuits, and the power supply. All of these sections must be separate from each other on the PCB. Additionally, the 1.2 GHz circuit components should be much closer together than the wavelength of the RF signal to avoid having the traces act as antennas. The wavelength of a 1.2 GHz RF wave through the PCB medium is approximately 15cm, so the traces should be no longer than about 1.5cm.

The upper board contains two major sections, which are the power circuitry and the PIC microcontroller. These sections need to be separate from each other in the PCB design [1]. This board will be physically placed to the side of the motor shaft and provided power through slip rings mounted on the shaft. Therefore, the power connector needs to be placed in the middle of one side of the PCB. The header connecting this PCB to the LED post PCB should be near the middle of the other side.

The upper and lower boards are connected using a 2.4 GHz wireless transceiver. The transceiver on the upper board should be mounted on the bottom side and the transceiver on the lower board should be placed on the top side so that there is a reliable line-of-sight communication link between the two. These transceivers should also be placed close to the axis of rotation to reduce the magnitude of a frequency shift due to the Doppler Effect. The percentage shift in frequency due to the Doppler Effect has been calculated to be 0.0000014%, which is negligible.

The LED post PCB contains LED drivers and RGB LEDs. The LEDs should be as close together as possible, while still allowing for soldering, in order to increase clarity of the final image. The LED drivers should be on the back of the PCB so that the only thing on the front of the PCB is the LEDs.

The primary concern with the transmitter, which is the final PCB, is that it should be as small as possible so that a user can easily carry it with them. Additionally, the amplifier and the antenna must be close together to provide the best possible signal.

Another concern is signal routing and trace sizing. The upper board and lower board both contain two surface mount voltage regulators. These voltage regulators require a series of vias underneath for cooling purposes. This means that no signals can be routed underneath [2][3]. Also, both the upper board and lower board have sections with high voltage and current. The slip rings transmit a 20V signal with a current draw that could reach 3A. These traces need to be large and spaced away from other traces to avoid arcing and trace burnout. Another area that requires a large trace is the power for the LED board. At maximum current draw, the LEDs could take up to 1.4A to power [4]. These traces should be at least 25 mils wide to carry this much current.

A final concern is EMI reduction. The project contains a number of noise sources that must be dealt with. The main sources are the 1800 RPM (30Hz) AC motor and slip rings, the 1.2 GHz RF circuit, the 550 KHz switching regulator, and the 8 MHz SPI on the PIC microcontroller. The signals received by the 1.2 GHz RF circuit are sent through a bandpass filter to reduce the effect of any other RF frequencies in the air. These areas of the PCB are also well grounded through a network of vias to avoid sending any noise through the ground planes. The transmission lines will also have a conductive metal casing over them to provide shielding from any electric noise. The AC motor and slip rings produce a significant amount of EMI. However, this noise is in a very low frequency range compared to the other components in the project design. The switching regulator alone has a much higher frequency of operation and can easily provide a reliable output voltage when faced with this low frequency noise. Additionally, a large capacitor is placed on the upper board to filter out the noise inherent in the slip rings. The 8 MHz SPI, which is used to send data to the LEDs, has the potential to generate a non-negligible amount of noise because of the large amount of current it is switching at a high frequency. However, this section of the project is located far away from other components, so the final product should not be affected by this noise.

3.0  PCB Layout Design Considerations - Microcontroller

The PIC24F microcontroller that is running on the upper-board has specific requirements for the placement of the bypass capacitors as well as the oscillator circuit [5]. The micro needs one bypass cap on each side of the SOIC package in order to filter unwanted electrical noise on the power line. They must be as close to the power and ground pins as possible, so they are located on the bottom side of the PCB directly beneath the microcontroller. The oscillator must also be located close to the pins of the microcontroller so that the crystal can supply sufficient gain without overdriving the crystal [6]. The PIC24F is a fairly low power device [7] and will be primarily driving CMOS logic except for the LEDs that can be used for debugging, so the power and ground traces of the microcontroller are just the standard 0.015” trace width. The PIC24F has 5 signals that it needs to receive from the 2.4 GHz RF transceiver and 4 signals that it needs to output to the LED drivers on the LED post. There are also headers that will connect to the IR LED and phototransistor that will be used for speed feedback. The LED and photo transistor cannot be mounted to the PCB because they must be located under the bottom support disc and reinforced for safety at that speed.

4.0  PCB Layout Design Considerations - Power Supply

The largest considerations for the PCB layout of the power supply include trace width and clearance of the higher voltage traces as well as part placement for the rectifier and AC power. The power input of the circuit is a 12.6Vrms voltage from a power transformer that is then rectified and filtered. The rectifier dissipates 6 watts and needs a heat sink so this part must be placed near the side of the lower PCB. The rectified voltage is around 18 Vdc so there needs to be more trace clearance around these traces. The ARM microcontroller is connected to a relay so that it can turn the drive motor on and off. This relay will have a 120V 60Hz line connected to it so it is critical to keep this portion of the PCB isolated from the rest of the DC power.

The switching power supplies also require a specific circuit layout. The TI 5V buck regulator has a POWER PAD™ on the bottom of the chip that must be thermally and electrically connected to a ground plane underneath the chip and have a least 4 vias [2]. The ground plane and vias are primarily for thermal dissipation. Another concern of the buck regulator is the distance of the inductor, bulk output capacitor and feedback line. These make an RLC circuit which could potentially have a resonance close to the switching frequency and cause instability in the output.

The transmitter is a single tone 1.2 GHz oscillator and amplifier that is powered by 2 Alkaline or rechargeable NiMH AA batteries. The logic of the transmitter is 5V so a Linear Technology boost converter is used to get a clean voltage to drive the RF components [8]. The boost converter will be converting 2-3 Vdc from the batteries to 5 V to use on the logic which will result in a fairly large current ripple in the inductor of the boost converter.

5.0  PCB Layout Design Considerations – RF Circuits

The position detection system of the POV device works on a 1.2GHz RF beacon and phase detector. The circuit layout of the phase detection circuit is absolutely critical to its functionality and accuracy. The four antennae connect to an SMA connector that is attached to the side of the PCB which is connected to a 1.2 GHz band pass filter and a 50 Ω transmission line. Of the four RF signals, two of them must go through a 90° phase shift which is achieved by adding a “U” shape to the 50 Ω transmission line of the corresponding signals. These signals are then fed into the phase detectors where the phase is output as an analog voltage. The transmission lines must have a ground plane beside and below the signal line to function similar to a coaxial line. The two ground planes will be connected by putting at least one via per square centimeter (about 7% of the wavelength through FR4) to prevent the transmission line from functioning as an antennae and adding more noise to the circuit.

The 1.2 GHz RF transmitter must be designed so that the noise sensitive RF amplifier will not pick up noise from the 5 V boost converter. The boost converter has a 155 KHz internal oscillator [7] and this should be as far from the RF circuit as possible on the transmitter PCB.

6.0  Summary

The POV Machine utilizes four PCBs that each have special layout considerations to take into account. The layout was designed to reduce EMI effects and prevent unwanted noise from disrupting the integrated circuits. The highly sensitive RF circuitry was placed far from potential noise sources such as a voltage regulator to prevent distortion in the RF signals. Special considerations for the power supply were taken into account to properly size the power traces, as well as provide a thermal release with the use of heat sinks.


List of References

[1]  Motorola Technical Staff, System Design and Layout Techniques for Noise Reduction in MCU-Based Systems, Motorola, 1995

[2]  Texas Instruments Technical Staff, Switching Voltage Regulator, Texas Instruments, 2004

[3]  Texas Instruments Technical Staff, LDO Voltage Regulator, Texas Instruments, 1999

[4]  Cree Technical Staff, RGB LEDs, Cree, 2009

[5]  Microchip Technical Staff, Microcontroller Oscillator, Microchip, 2008

[6]  Microchip Technical Staff, Basic PICmicro Oscillator Design, Microchip, 2002

[7]  Microchip Technical Staff, PIC Microcontroller, Microchip, 2008

[8]  Linear Technology Technical Staff, Linear Technology Boost Converter, Linear Technology, 1993

-6-